Typical vehicles such as automobiles have increasingly been equipped with an airbag device to inflate/deploy an airbag inside the vehicle in an emergency such as at a collision of the vehicle so as to absorb an impact exerted on an occupant. Such an airbag device typically includes an airbag and an inflator that supplies gas to the airbag. The airbag is normally housed in a structure of the vehicle in a folded state whereas the airbag is inflated/deployed in an emergency.
For example, a side airbag that is inflated/deployed between an occupant and a door portion is embedded in a rear portion (seat back) of a seat on which the occupant is seated. At a collision of the vehicle, the side airbag breaks through the seat or pushes aside the seat to be released forward inside the vehicle. Methods have been developed for such a side airbag device to prevent a rise of the internal pressure of the airbag in the case where the airbag is hindered from being normally inflated/deployed (see, for example, Japanese Unexamined Patent Application Publication No. 2007-50848 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-535121).
An airbag device described in Japanese Unexamined Patent Application Publication No. 2007-50848 includes a vent hole formed at a corner portion of a lower bag and a cloth member that has such a size and a shape as to be capable of closing the vent hole and that has a through hole. Before the lower bag is deployed into a predetermined shape, the position of the through hole coincides with the position of the vent hole to allow gas to be discharged from the lower bag. On the other hand, when the lower bag is deployed into a predetermined shape, the tension exceeding a predetermined value is exerted on the cloth member to break a tear seam portion, so that the through hole moves forward and the cloth member closes the vent hole.
An airbag device described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-535121 includes a side panel and a closing panel. The side panel serves as an outer surface of the side airbag and defines a vent hole for releasing gas to the outer side of the side airbag. The closing panel is disposed around the vent hole and configured to keep the vent hole open to release the gas while the side airbag is inflated until the side airbag is substantially fully inflated.